1. Field of the Invention
The invention relates to a device for determining thermal properties of a medium, in particular humidity sensors, and to a method for the measurement of humidity in a medium, in particular of soil.
2. Description of Related Art
Whilst the measurement of the relative air humidity today is possible in a very simple manner by way of conventional as well as electronic sensors, the measurement of the humidity in mixtures however, in particular of ground or plant substrates, is still problematic.
Electronically functioning humidity sensors mostly operate on the basis of dielectric layers absorbing moisture, which have a dielectric constant which is greatly dependent on the water content. These layers are introduced between two electrode layers which form a plate capacitor and which measure the change of capacitance of this capacitor by way of an applied alternating voltage.
This principle is very suitable for measuring air humidity, but may not however be applied for determining the ground humidity. On the one hand, the sensor would have to be sunk into the ground, wherein a direct gas-permeable connection between the ground and the sensor, as well as a good aeration of the sensor must be guaranteed for a reliable evaluation of the humidity. A direct, gas-permeable connection however creates enormous difficulties. Since the ground may not only be damp, but also wet, the gas connection mostly conceived as a porous filter comes into direct contact with the fluid and becomes blocked. The filter also becomes infested with ground bacteria and ions may migrate through the filter into the sensor region, and destroy it. Furthermore, the sensor including the activation and evaluation electronics is expensive and thus not suitable for large-scale application.
Other methods, as a measurement variable, use the change in the electrical resistance with an increasing humidity of a suitable material absorbing water, such as plaster or nylon, which is brought into contact with the surrounding soil, microwaves of a certain frequency which are locally radiated into the ground, or capacitative sensors which determine the dielectricity constant of the surrounding soil by way of an electrical alternating field. Apart from further disadvantages, all these methods entail high costs and also, to some extent, a considerable technical effort.
For this reason, tensiometer systems are mainly used for measurements of ground substrates. A porous clay-like cell saturated with water is connected by way of a plexiglass tube filled with water to a manometer in an airtight manner, for measuring a vacuum. Due to the contact of the cell with the ground, the existing vacuum of the ground water is transmitted via the clay-like cell and the water filling to the manometer, and may be read there. With an increasing humidity of the ground, the suction tension of the ground water increases, and this is displayed by the manometer as a vacuum.
This principle is used today, in particular in the field of ground humidity measurement in nurseries etc., but also with irrigation systems for balcony and terrace plants, with the following disadvantages. The porous measurements bodies need to be in direct connect with the surrounding substrate over the whole surface, and thus may easily become blocked or also calcified for example with calciferous water. For this reason, they need to be controlled and exchanged again and again. The water supply required for this needs to be controlled and regularly topped up. Furthermore, the manufacturing costs of such a sensor are relatively high and lie in the region of 30-50 Euros. A direct electronic evaluation entails a considerable expense, since for this, the sagging of the membrane needs to be converted into an electronic signal.
Apart from sensors which make use of neutron backscatter and absorption as a measurement principle, there exist yet a series of further sensor principles. The principle of measurement of the thermal conductivity of the ground is also to be found amongst these. This method however has not asserted itself up to now, despite various developments. Thus WO9013812 describes a method with which the soil is located in the inside of a cylinder which is provided with a heating element and a temperature sensor. The heating element produces a heat pulse whose heating effect on the soil is measured with the temperature sensor, and is evaluated as a measure of the humidity. In particular, the thermal conduction is determined to a great extent by the nature and size of the contact surface between the soil and the sensor, which may not be clearly definable. The nature and porosity of the soil also plays an important role.
A further method with the measurement of thermal conductivity is described in JP03061845A2. Thereby, a heating element and a temperature probe are arranged at a certain distance along a measurement rod. The heating element heats the surrounding soil and transports a part of the thermal energy via the surrounding soil to the temperature probe, which indicates a temperature increase. One may deduce the water content of the surroundings by way of this. This sensor too is unreliable since the border surface between the heating element and the soil, as well as between the temperature probe and the soil, is not defined.
In order to avoid the disadvantage of the contact surface between the soil and the sensor, which may not be clearly defined, a method has been developed in DE2536777 with which the temperature probe is surrounded with an artificial standard soil, so that the border surface from the probe to the surroundings is better defined. Thereby, the temperature increase is measured before and after a short heating pulse, and the difference is used for determining the moisture content of the soil. It is also known for the standard soil to be replaced by porous ceramics.
This method has the disadvantage that the difference between the start and end temperature depends on the water content only to a limited extent, i.e. the sensitivity of such a method is very poor. Furthermore, the standard soil itself has a high thermal capacity and thermal conductivity, which influences the measurement.
An inexpensive silicon chip is described in the document WO 98/52027, which is also used as a humidity sensor. In this, the surface of a silicon membrane is covered with an absorbent material in order to lead moisture to the sensor from the surroundings. The measurement surfaces are very small, so that a transition resistance sensor-soil, with a heterogenic soil mixture, is completely random. Furthermore, on measuring, the thermal conductivity of the soil through the thin material dominates, so that the measurement becomes unreliable on account of an undefined (thermal) composition of the surrounding soil.
The document DE 43 40 775 likewise recognizes the fact that a good contact between the sensor housing and the soil is important. Thereby, it is suggested to press the housing walls onto the soil by way of pressure. Thereby however, the substrate is locally compacted and its thermal properties change. Furthermore, soil is often compressible and evades pressure. If the substrate subsequently dries, the sensor may detach from the substrate, which causes a greatly changed thermal resistance on the border surface.
Other measurement methods such as described in U.S. Pat. No. 5,287,734 for example, use porous ceramics which are arranged between a heating element and a temperature probe. If the ceramics become soaked with the water of the surrounding soil, the thermal conductivity value of the ceramics changes, which gives a conclusion with regard to the water content of the soil. The disadvantage of this method lies in the fact that the ceramics are not enriched with water to an equal extent as the surrounding soil, since the distribution of water-suctioning pores between the soil and the ceramic is very different. Furthermore, pockets of air are formed, since the air may no longer escape from the ceramic body. The sensitivity is poor since only a fraction of the volume of the ceramics is present in the form of pores. The thermal conductivity of the surrounding ground has a great effect on the measurement, due to the relatively high thermal conductivity of the ceramics.
For the above mentioned reasons, and in particular on account of the high costs, none of the methods which are based on the thermal measurement principle have asserted themselves until now.